1. Field of the Invention
The invention relates to a device for the simultaneous double-side grinding of a workpiece in wafer form.
2. Background Art
Devices which are used for the simultaneous grinding of both sides of workpieces in wafer form, for example semiconductor wafers or in particular silicon wafers, are known from the prior art. They are generally known as double-side grinding machines. A further variant of double-side grinding is the process known as double disk grinding, or DDG for short.
DDG machines according to the prior art, as are described, for example, in JP2000-280155A and JP2002-307303A, have two grinding wheels which are positioned opposite one another and the axes of rotation of which are arranged collinearly. During the grinding operation, a workpiece in wafer form which is positioned between the grinding wheels is machined on both sides simultaneously by the two grinding wheels rotating about their axis, while the wafer is held in position by an annular holding and rotation device and at the same time rotated about its own axis. During the grinding operation, the two grinding wheels are advanced in the axial direction until the desired final thickness of the workpiece has been reached.
The holding and rotation device may, for example, comprise friction wheels which engage the edge of the workpiece. However, it may also be a device which surrounds the workpiece in the form of a ring and engages in a score, groove or notch which is present at the periphery of the workpiece. A device of this type is generally known as a notch finger. To machine the entire surface of the workpiece, the workpiece is guided in such a way relative to the grinding wheels that the abrasive grinding segments of the grinding wheels describe an orbit which runs constantly over the center of the workpiece.
In this arrangement, the workpiece is not generally in a fixed position, but rather is held axially in place by two devices for hydrostatic bearing, referred to below as hydropads, arranged on either side of the workpiece. Devices of this type are described in JP2002-280155A. According to the prior art, those surfaces of the two hydropads which face the workpiece are of planar configuration and oriented parallel to one another. Each hydropad comprises a plurality of hydrostatic bearings, between which grooves for discharging the medium used for the hydrostatic positioning (referred to below as the “hydro-bearing medium”) and the grinding coolant are arranged.
In each case one or more measuring sensors are integrated in the hydropads, allowing the spacing between the surface of the hydropads and the workpiece surface to be measured during the grinding operation. This spacing measurement is usually carried out with the aid of dynamic pressure tubes as a pneumatic dynamic pressure measurement. The dynamic pressure tubes are designed as simple bores in the edges of the hydrostatic bearings, which form the guide surfaces. To enable the spacing between the hydropads and the workpiece to be measured as close as possible to the location of grinding machining, the dynamic pressure tubes are generally arranged close to that edge of the hydropads which adjoins the grinding wheels.
This spacing measurement is part of a control circuit which is responsible for centering the workpiece between the hydropads. The actuator of this control circuit is the pair of grinding wheels, which are axially displaced with respect to their own axis of rotation as a function of the result of the dynamic pressure measurement, in such a manner that the measured dynamic pressure and therefore the spacing between the workpiece and the hydropad is equal on both sides of the workpiece.
With a bearing arrangement of this nature for a workpiece in wafer form, the following problems arise during the grinding operation, which lead to a deterioration in the geometry of the machined workpiece and in particular in the geometry parameter known as nanotopography:    1. The dynamic pressure measurement is carried out during the grinding operation. This means that the hydro-bearing medium and grinding coolant, which is laden with workpiece chips, pass into the region of the dynamic pressure tubes and can disrupt the dynamic pressure measurement. Consequently, the workpiece is not accurately positioned equidistantly between the hydropads during the grinding operation.    2. Different abrasive characteristics of the two grinding wheels result in different compressive stresses in the surface of the workpiece (subsurface damage), which leads to curvature of the workpiece. This curvature is generally rotationally symmetrical. This in turn means that partial regions of the workpiece will not be located centrally between the hydropads. Since the hydrostatic bearing counteracts this tendency of the workpiece, the workpiece is pressed unevenly onto the two grinding wheels, and a correspondingly rotationally symmetrical curvature is ground into the workpiece.